Phase transition and electronic structure evolution of MoTe2 inducedby W substitution

Santos, E. J. G. (2018). Phase transition and electronic structure evolution of MoTe2 induced by W substitution.Physical Review B, 98, 144114-144120. https://doi.org/10.1103/PhysRevB.98.144114

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Phase transition and electron structure evolution of MoTe2 induced by W substitution

Wencan Jin,1, ∗ Theanne Schiros,2 Yi Lin,1 Junzhang Ma,3 Rui Lou,4 Zhongwei Dai,5 Jiexiang Yu,5 Daniel Rhodes,6

Jerzy T. Sadowski,7 Xiao Tong,7 Tian Qian,3 Jerry I. Dadap,1 Shancai Wang,4 Jiadong Zang,5 Karsten

Pohl,5 Hong Ding,3 James Hone,1 Luis Balicas,6 Abhay N. Pasupathy,1, † and Richard M. Osgood, Jr1, ‡

1Columbia University, New York, New York 10027, USA2Fashion Institute of Technology, New York, New York 10001, USA

3Beijing National Laboratory for Condensed Matter Physics,and Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China4Department of Physics, Renmin University of China, Beijing 100872, China

5University of New Hampshire, Durham, NH 03824, USA6Florida State University, Tallahassee, Florida 32306, USA

7Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, New York 11973, USA

The transition metal dichalcogenide compounds MoTe2 and WTe2 are polymorphic with bothsemiconducting and metallic phases. The thermodynamically stable phase of WTe2 at room tem-perature is monoclinic and metallic, and displays a wide range of interesting phenomena includingtype-II Weyl fermions, titanic magnetoresistance and superconductivity in the bulk, and quantumspin Hall insulator behavior in the monolayer. On the other hand, the stable phase of MoTe2 atroom temperature is a trigonal prismatic semiconductor that has a direct gap in the monolayerwith strong spin-orbit coupling. The alloy series Mo1-xWxTe2 thus offers the possibility of tuningthe structural and consequently electronic phases via tuning of the composition. Here, we reportcomprehensive studies of the electronic structure of Mo1-xWxTe2 alloys using angle-resolved photoe-mission spectroscopy and first principle calculations as a function of composition. We find a sharpboundary at room temperature between the monoclinic and trigonal prismatic phases at x = 0.10from structural characterization. We also show that by compositional tuning it is possible to con-trol the band inversion in this series of compounds, with important consequences for the topologicalsurface states.

I. INTRODUCTION

Transition metal dichalcogenides (TMDCs, MX2,M=Mo, W; X=Se, Te) are polymorphic with differentcrystal structures, including trigonal prismatic 2H phase(Fig. 1(a)), monoclinic 1T’ phase, and orthorhombic Tdphase (Fig. 1(d)). These phases provide an importantplatform for exploring exotic physics and novel deviceapplications. The semiconducting 2H -phase TMDCs inmonolayer form consists of a layer of hexagonally ar-ranged transition metal atoms sandwiched between twolayers of chalcogen atoms. In monolayer 2H -TMDCs,the sizeable direct band gap [1–3] and valley degree offreedom [4–6] make this phase remarkably appealingfor electronics [7], and spin- and valley-tronics devices[8, 9]. Recently, the metallic 1T’ and Td phase haveattracted interest due to the presence of band inversionin these phases, making them important candidatesfor realizing novel topological quantum phenomena.The semimetallic 1T’ phase exhibits a distorted oc-tahedral structure with an inclined stacking angle of∼93.9◦, which retains a centrosymmetric P21/m spacegroup. In contrast, in the orthorhombic Td phase, thestacking angle is exactly 90◦, which breaks inversion

∗ Current address: University of Michigan, Ann Arbor, MI 48109,USA† apn2108@columbia.edu‡ osgood@columbia.edu

symmetry (space group Pmn21) [10]. The Td phasedisplays a number of unique electronic properties inboth the bulk and monolayer forms. Bulk crystals ofboth Td-WTe2 [11, 12] and Td-MoTe2 [13] display alarge, non-saturating magnetoresistance, possibly dueto electron-hole compensation. Bulk Td-WTe2 [14],Td-MoTe2 [15, 16] as well as some of their alloys [17] arealso known to be type-II Weyl fermions. These type-IIWeyl fermions are characterized by touching pointsbetween electron- and hole-pocket with strongly tiltedWeyl cones [14] in the bulk, and Fermi-arc states onthe surface. A number of angle-resolved photoemissionspectroscopy (ARPES) studies of the electronic structureof Td-WTe2 [18–20], MoTe2 [21–24] and MoxW1-xTe2 onthe W rich side [25, 26] have found evidence for thesefeatures. Finally, in monolayer form, the Td-TMDCs aretwo-dimensional topological insulators that display thequantum spin Hall effect [13, 27].

The rich electronic phenomenology associated with thesemiconducting and metallic phases of these compoundshas spurred intense interest in achieving precise controlof transitions between these phases. Such phase engi-neering of TMDCs has recently been attempted using avariety of tuning parameters such as temperature, strain,chemical doping and electrostatic doping [13, 28–32].Achieving reversible control of the phase transition isimportant for on-demand topological properties as wellas for the development of technological applicationssuch as phase-change memory devices [33, 34]. One of

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the chief difficulties in achieving this reversible controlin the parent compounds MoTe2 and WTe2 is thatthey are fairly stable at room temperature. In thiscontext, alloying has been shown by some of us to playan important role in structural phase control (referenceprevious work). Briefly, at room temperature, thethermodynamically stable phase of MoTe2 is the semi-conducting 2H polymorph. In contrast, the stable phaseof WTe2 is the semimetallic Td polymorph. By usingchemical alloying to produce the series Mo1-xWxTe2, weshowed the existence of a phase transition at aroundx=0.08. We expect that as we approach the criticaldoping that the alloys become sensitive to dynamicaltuning parameters such as strain and electrostaticdoping, opening up new possibilities for structuralcontrol. As we approach this critical doping, we mustunderstand the electronic structure of the alloys oneither side of the phase transition in details. In thiswork, we achieve this by performing synchrotron-basedARPES measurements of Mo1-xWxTe2 as a functionof W concentration, supported by structural charac-terization and density functional theory. The majorfindings of our study show that (i) the phase transitionfrom 2H to Td appears on the Mo-rich side (criticalW concentration xc ∼ 0.10) of the alloy compositionpoint, in contrast to the previously predicted value of0.33 [35]; (ii) W doping results in a downward shift ofconduction band minimum, thus enhancing the bandinversion; and (iii) demonstration via density functionaltheory calculation that interlayer coupling in this ma-terial is weaker than in widely studied TMDCs like MoS2.

These predictions for type-II Weyl fermions havetriggered a series of angle-resolved photoemission spec-troscopy (ARPES) studies of the electronic structureof Td-WTe2 [18–20], MoTe2 [21–24] and MoxW1-xTe2on the W rich side [25, 26]. These measurementshave shown that the size of the Fermi arc in WTe2 isextremely small due to the small separation of the Weylpoints, while the Fermi arc of MoTe2 is more extendedin momentum space, indicating the topological strengthof the latter is more robust. Therefore, Mo1-xWxTe2alloy on the Mo rich side, in comparison with the W-richcounterpart, is more appealing for investigation of theelectronic structure evolution and tunable topologicalstrength. In addition, MoTe2 exists 2H, 1T’ and Tdphases, while WTe2 has commonly been observed in theTd structure. It is technically more efficient to inducea phase transition in MoTe2 by W doping, instead ofdoping Mo to WTe2. To this end, Mo-rich Mo1-xWxTe2alloys makes an ideal playground for exploring the struc-tural phase transition and electronic structure evolution,and thus there is a pressing need for a comprehensiveexperimental investigation.

In this work, we report electronic structure evolutionof Mo1-xWxTe2 as a function of W concentration usingsynchrotron-based ARPES and in concert with density

functional theory calculations. The major findings ofour study show that (i) the phase transition from 2H toTd appears on the Mo-rich side (critical W concentrationxc ∼ 0.10) of the alloy composition point, in contrastto the previously predicted value of 0.33 [35]; (ii). Wdoping results in a downwards shift of conduction bandminimum, thus enhancing the band inversion; and (iii)demonstration via density functional theory calculationthat interlayer coupling in this material is weaker thanin widely studied TMDCs like MoS2.

II. RESULTS AND DISCUSSION

The synthesis method for our crystals has beendetailed in Ref.[36]. Prior to our ARPES measurements,the composition of the alloys was first determined withx-ray photoemission spectroscopy (XPS) as describedin the Methods section. The crystalline structure ofMo1-xWxTe2 alloys was then investigated using selected-area low energy electron diffraction (µ-LEED) at roomtemperature. The well-defined hexagonal µ-LEEDpattern (see Fig. 1(b)) acquired from x = 0.08 alloy,demonstrates that the alloys with x < xc crystallize inthe 2H-phase. In comparison, the rectangular µ-LEEDpattern (Figs. 1 (e)) for a crystal of composition of x= 0.16 shows that moderate W substitution (x > xc)has stabilized the Td-phase at room temperature. Notethat multiple locations were surveyed across the samplesurface, and no phase coexistence was observed. Inaddition, to examine any major changes in crystal struc-ture within surface layers, we determined the surfacestructure of these Mo1-xWxTe2 alloys using dynamicalLEED calculations[37–39]. The measured LEED-I-V(blue solid curve) for the (00) diffraction beam andcalculated I-V (black dashed curve) are shown in Fig. 1(c) and (f). The optimized surface structure for 2H -and Td phase were obtained by fitting the calculated I-Vcurves to the measured ones, and the results indicatethat surface structure while distinct from that in thebulk is not significantly different enough to change theelectronic structure.

ARPES measurements were then used to investigatethe electronic structure of Mo1-xWxTe2 alloys; tosharpen the spectra the measurements were made atlow temperature, typically 50 K. Figure. 2 (a) showsthe ARPES bandmap of 2H -Mo0.94W0.06Te2 alloy alongthe K-Γ-K high-symmetry direction of the surfaceBrillouin zone (see inset). The corresponding integratedspectrum (Fig. 2 (b)) shows that the main band featuresare derived from the Mo dz2 and Te pz orbitals. Theconduction bands were not observed up to 1 eV abovevalence band maximum (VBM), confirming 2H -phaseis semiconducting with a gap size > 1 eV. The bandfeatures are further displayed as corresponding energydistribution curves (EDCs) plots in Fig. 2 (c). A

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(a) (b) (c)

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FIG. 1. Crystalline structure of 2H - and Td- Mo1-xWxTe2 crystals. (a) Schematic of 2H -phase atomic structure in topview (left) and sideview (right), (b) LEED pattern, and (c) µ-LEED I-V curve for 2H -Mo0.92W0.08Te2.(d) Schematic of Td-phaseatomic structure in topview (left) and sideview (right), (e) LEED pattern, and (f) µ-LEED I-V curve for Td-Mo0.84W0.16Te2.Blue spheres: Mo/W atoms; yellow spheres: Te atoms.

parabolic fit of the topmost valence band yields a holeeffective mass at Γ of 3.77m0 (where m0 is the electronmass), which is even larger than that of monolayerMoS2 (∼ 2.4m0). As revealed, the thickness-dependentelectronic structure of MoS2 [2], the topmost valenceband at Γ decreases in energy with decreasing interlayercoupling strength, which leads to increasing hole effectivemass. The remarkably flat topmost valence band impliesrather weak interlayer coupling in 2H -Mo0.94W0.06Te2alloy.

In contrast to the semiconducting 2H phase, theARPES bandmap (Fig. 2 (e)) of the Td-phase (x = 0.2)along Y -Γ-Y (Fig. 2 (d)) high symmetry direction showsa metallic nature, in which a hole band (yellow arrow,α) and electron pocket (white arrow, β) both cross theFermi level. In addition, the surface state (red, SS)protrudes the electron pocket and almost overlaps withthe hole band, which indicates that it is derived froma type-B surface [19]. These band features are furtherdisplayed in the EDCs plot (Fig. 2 (f)). Figure 2 (g)shows the stack of constant-energy maps. In particular,a palmier-shaped hole pocket and an almond-shapedelectron pocket are observed in the Fermi surface (E= EF ) map. The sizes of hole and electron pocketsincrease and decrease with increasing binding energy,respectively. We have comprehensively measured theelectronic structure evolution of the Td phase as afunction of W concentrations. A side-by-side comparisonof the electronic structure is made between x = 0.16,

x = 0.20, and x = 0.27. As shown in the ARPESbandmaps and corresponding second derivative plots inFig. 3(a)-(c), the overlap in energy between valence andconduction band decreases with increasing W concentra-tion. Such overlap is characterized by the energy postionof the conduction band minimum (CBM), as shown inthe EDC plots (Fig. 3(d)). The CBM of x = 0.16 islocated at ∼ 50 meV, which is comparable with that ofpure Td-MoTe2 (60 meV) [21]. As the W concentrationincreases, the CBM shifts towards the Fermi level,and in the x = 0.27 alloy, the overlap is significantlysuppressed, given the large content of Mo relative to W.While we do not directly visualize the Weyl crossingsin this data set, it is clear that the change in theband inversion also tunes the separation between Weylpoints and consequently the surface state band structure.

To further investigate the band inversion observedhere, we use density functional theory calculations tostudy the evolution of the electronic structure with al-loying. Our previous photon energy-dependent ARPESstudies [36] and theoretical calculations [40] have re-vealed that the dispersion along kz direction showstwo-dimensional character. Here, we employ a modelat the 2D limit by considering only one unit cell (1UC)thickness of the Td structure (see Fig. 1(d)). Figure 4(a)shows the calculated hole band and electron pocket asa function of W concentration. As W concentrationincreases, the hole band slightly sinks down while theelectron pocket rapidly shifts upwards. Our 1UC model

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1.00.50.0Intensity (a. u.)

Mo dz 2

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FIG. 2. Electron structure of Mo1-xWxTe2 alloys across critical W concentration. Electronic structure of 2H -Mo0.94W0.06Te2 alloy (a) ARPES bandmap along K-Γ-K high symmetry direction, inset shows the surface Brillouin zone (b)Integrated spectrum and (c) EDCs plot of ARPES bandmap shown in (a). Electronic structure of Td-Mo0.80W0.20Te2 alloy(d) the bulk Brillouin zone (BZ) and projected (001) surface Brillouin zone (SBZ), (e) ARPES bandmap (hν = 24 eV) alongY -Γ-Y high symmetry direction, (e) EDCs plot of band features near Fermi level (EF ), and (g) stack of constant energy maps.

-0.5

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Mo0.73W0.27Te2

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Mo0.84W0.16Te2(a) (b) (c)

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nsity

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.)

-100 0 100

x = 0.16

x = 0.20

x = 0.27

(d)

FIG. 3. Electronic structure evolution for different Td-Mo1-xWxTe2 alloys with W concentrations. ARPESbandmap (left) and corresponding second-derivative intensity plot (right) of Td-Mo1-xWxTe2 alloys along Y -Γ-Y high symmetrydirection with (a) x = 0.16, (b) x = 0.20, and (c) x = 0.27. (d) Corresponding EDCs across conduction band minimum.

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capture the measured band inversion evolution verywell. This result further confirms the 2D character ofthe electronic strucrure. First principle calculations canalso give us insight into the structural stability of the 2Hand Td phases as a function of alloying. To clarify thisquestion, we first investigated both the 2H and Td phaseof the bulk and monolayer structures of pure MoTe2 andWTe2. To directly compare the total energy between 2Hand Td phase with different W concentration, we employan expanded supercell containing 12(24) Mo/W atomsfor 1UC (bulk) cases. This supercell can be regardedas a 3×2 supercell of the primitive cell for Td phase.The energy difference per Mo/W atom between 2H andTd are shown in Fig. 4(b). The energy difference as afunction of doping value x is nearly a linear relationship,which leads to a phase transition point at x = 0.25for bulk and x = 0.13 for 1UC case. Here, the phasetransition point predicted by bulk model is close tothat of previous study, while the 1UC model moreaccurately capture the measured critical doping value(xc ∼ 0.10), which further confirms that Mo1-xWxTe2crystal is close to a 2D system rather than a vdW system.

III. CONCLUSION

In conclusion, we investigate the structural phase tran-sition and electronic structure evolution of Mo1-xWxTe2alloys using ARPES and DFT calculations. Our resultsshow that a critical W concentration of x ∼ 10% triggera phase transition from 2H to Td phase, and in the Tdphase, topological strength is significantly suppressed byincreasing W doping. The phase transition point andelectronic structure evolution is captured well by theDFT calculations using a 1UC model, indicating the 2Dcharacter of this material due to weak interlayer coupling.

IV. METHODS

X-ray photoemission spectroscopy Stoichiome-try of alloys were determined by x-ray photoelectronspectroscopy (XPS) at the RHK Technology UHV7500 facility at Center for Functional Nanomaterials atBrookhaven National Laboratory. The samples werecleaved in-situ with a pressure < 10−9Torr. The XPSexperiments were conducted at room temperature usingMg Kα line with an energy resolution of 0.9 eV.

Angle-resolved photoemission spectroscopy.ARPES measurements were performed at the Dreamlinebeamline of the Shanghai Synchrotron Radiation Facility(SSRF) with a Scienta D80 analyzer. The samples weremeasured at 40 K with a base pressure < 5 × 10−11

Torr. The ARPES data were collected within 12 hoursafter cleavage, during which time no signature of surface

degradation was observed. The energy and angularresolutions were set to 15 meV and 0.2◦, respectively.(Need input from Theanne about Stanford SynchrotronRadiation Lightsource.)

Low energy electron microscopy. µ-LEED mea-surements were performed at the Center for FunctionalNanomaterials, Brookhaven National Laboratory usingan ELMITEC AC-LEEM system. In this system, thesample was cleaved in situ at room temperature. Thespatial resolution is <3 nm in the LEEM mode. Theelectron-beam spot size in the µ-LEED mode is 5 µm indiameter.

First-principles electronic structure calculation.Density functional theory calculation was carried outusing a VASP package [41] with a projector augmentedplane-wave potential [42]. The exchange-correlationenergy was described by the generalized gradient ap-proximation in Perdew, Burke, and Ernzerhof (PBE)form [43]. The Brillouin zone of the orthogonal unit cellof Td-Mo1-xWxTe2 were sampled by a 7×12×3 k -pointmesh. The energy cutoff was set to 440eV. Van der Waalsinteractions were incorporated within the Tkatchenko-Scheffler method [44]. Spin-orbit coupling was alsoincluded for structural optimization. All structures wereoptimized until the atomic force on each atom withboth Hellmann-Feynman and van der Waals terms weretaken into acount, is less than 1meV/A. For one unitcell case, a vacuum layer of 15 A is used to build 2D slabs.

Dynamical LEED Calculation. The codes fromAdams et al. [45], which were developed from theprograms of Pendry [46] and Van Hove and Tong [47],were used in the dynamical LEED calculations. Thelattice constants of WTe2 are a = 6.282 A, b = 3.496 A,c = 14.07 A [48]. The lattice constants of MoTe2 area = 6.335 A, b = 3.477 A, c = 13.883 A [15]. As thelattice contant difference between MoTe2 and WTe2 isless than 1.5%, we use the weighed average as the latticecontants of Mo1-xWxTe2 alloys. The Debye temperaturefor Mo1-xWxTe2 was set as 210K. The inner potential ofMo1-xWxTe2 is set as 10.1 eV. 12 (L = 11) phase shiftsare used in the calculation.

V. ACKNOWLEDGEMENTS

The LEEM/LEED research was carried out in part at theCenter for Functional Nanomaterials, Brookhaven Na-tional Laboratory, was supported by the U.S. Depart-ment of Energy, Office of Basic Energy Sciences, underContract No. DE-SC0012704. The work of R.M.O., J.D.,W.J. and Y.L. was financially supported by the U.S. De-partment of Energy under Contract No. DE-FG 02-04-ER-46157. In addition, Z.W.D., and K.P. were supportedby NSF DMR 1006863. R.L. and S.C.W. were supported

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(b) (a)

FIG. 4. (a) DFT calculated hole band and electron pocket with various W concentrations. (b) The calculated energy differencebetween 2H and Td phase as a function of doping value x of Mo1-xWxTe2 for both bulk and 1UC.

by the National Natural Science Foundation of China(No. 11274381). J.Z.M., T.Q., and H.D. were supportedby the Ministry of Science and Technology of China (No.2015CB921300, No. 2013CB921700), the National Nat-

ural Science Foundation of China (No. 11474340, No.11234014), and the Chinese Academy of Sciences (No.XDB07000000). (Need funding acknowledgement fromElton, Theanne and Prof. Pasupathy.)

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